This project is designed to provide information about the organization of neuronal systems in the mammalian spinal cord that are involved in the neural control of movement, using the isolated brain stem and spinal cord of neonatal mice in vitro. During FY 1999, we continued to study low-frequency synaptic depression (LFSD; 0.0125 - 2.0 Hz, 10 pulses) of dorsal root monosynaptic excitatory postsynaptic potentials (EPSPs) in lumbosacral spinal motoneurons. Using intracellular recording with conventional sharp micropipettes filled with 2M K acetate and 100 uM QX-314 to block sodium action potentials, we found that increasing bath temperature from 24 deg. C (room temperature) to 32 deg. C consistently reduces low-frequency synaptic depression, without concomitant changes in the amplitude of the control EPSPs. LFSD also systematically decreases as postnatal age increases from 1 to 12 days. The amplitude of the second response (R2) is also less than that of the average amplitude of the 8th, 9th, and 10th pulses (tail response). We found that the room temperature (23-24 deg. C) R2 and tail responses at all ages could be described as a sigmoidal function of postnatal age and stimulus frequency, suggesting that the data might be fit by a relatively simple model. We tested this sigmoidal function against three models of increasing complexity and found that a three process model fits the central tendency with remarkable precision. The final model includes two independent depletion processes that depress synaptic response and recover with the same exponential time course. These are assumed to involve transmitter and release site depletion, respectively. A third process that produces covert potentiation that decays with another exponential time course is needed to fit the data accurately. This is assumed to involve accumulation of residual calcium. The parameters of this model exhibit steady changes with increasing postnatal age. The model also fits the data obtained at 32 deg. C, at which all model parameters at any age show smaller values than at room temperature. Predictions of this model are being tested by manipulating extracellular calcium and with drugs that alter transmitter release and receptor desensitization. As noted in the Annual Report for FY1998, experimental work on the spinal cord of adult cats has been suspended indefinitely. During FY1999 we completed analysis of data tapes obtained in earlier experiments on the modulation of transmission of information through segmental interneurons to motoneurons during fictive locomotion. These data have been used to examine the organization of specific sets of last-order interneurons in reflex pathways that project directly to motoneurons, as well as the pattern of control of these pathways by the central pattern generator (CPG) for locomotion, which is located in the spinal cord. Two full papers in refereed journals were published and a third is in preparation. In addition, two invited reviews that deal with this work are in press. - motoneurons, interneurons, spinal circuits, locomotion, intracellular recording, synaptic potentials